The present invention concerns a method for controlling drive slip, and more particularly, to a method used in a vehicle driven by an internal combustion engine whose setting parameters are centrally controlled and in which the actual slip of the vehicle's driven wheels is continually recorded by a calculating unit and compared with the required slip, such that, in the case where the actual slip of at least two driven wheels on different sides of the vehicle deviates from the required slip by a certain slip threshold value, a signal formed in the calculating unit is emitted to a central control unit. On the appearance of this signal, at least one setting parameter of the internal combustion engine is altered by the central control unit to reduce the engine torque transmitted from the internal combustion engine.
A method of the foregoing type, generally speaking that is, is disclosed in DE 32 24 254 A1. The slip of the driven wheels is determined in a calculating unit from wheel rotational speed signals. If an excessive slip occurs, the engine torque transmitted from the internal combustion engine is lowered by a reduction in the quantity of fuel injected. The degree of reduction is determined in the calculating unit and is transmitted to the injection valves of the internal combustion engine via a central control device of the internal combustion engine, which can for example be a Bosch "L-Jetronic" brand device. The degree of reduction then depends on the wheel slip occurring in such a way that when different threshold values of the slip are reached, greater reduction in the quantity of injected fuel takes place in steps.
A disadvantage of this method is that the adjustment of the setting parameters of the internal combustion engine is transmitted from the calculating unit. Only the end stages of the central control device are used to control the setting parameters. The adjustment of the setting parameters of the internal combustion engine is, in this case, only specified as a function of the slip of driven wheels without taking account of the operating conditions of the internal combustion engine, such as its temperature.
It is also known, as shown in DE 34 35 869 A1 and DE 37 11 913 A1, to cause an automatic gearbox to change up on the occurrence of slip at driven wheels. Action also is taken on the setting parameters of the internal combustion engine, in addition to changing up the automatic gearbox, in order to reduce the engine torque of the internal combustion engine. From the disclosure in DE 34 35 869 A1, it is known that action can be taken on the engine control and, simultaneously, the automatic gearbox can be changed up on every reduction of the drive torque. From the disclosure in DE 37 11 913 A1, it is known that the automatic gearbox can be changed up on every reduction of the drive torque and, in order to improve the behavior with time of the reduction of the drive torque, action can be taken on the engine control in order to reduce the engine torque transmitted until the gear-changing of the automatic gearbox has been concluded.
Disadvantages arise with these known drive slip control systems in that, on the occurrent of an excessive slip of driven wheels, the automatic gearbox is always changed up. Increased slip of the driven wheels can, however, also occur for a short period when, for example, the vehicle drives over a small iced area, e.g., when there is a short-term reduction of the friction coefficient .mu.. Because this perturbation is only short-term, it is found desirable not to cause the automatic gearbox to change up immediately, particularly when the lower drive torque which can be transmitted because of the reduced friction coefficient .mu. can be achieved by reduction of the engine torque alone. In this situation, therefore, a superfluous gear-change of the automatic gearbox occurs, which leads to mechanical loading of the automatic gearbox because of this gear-change and, in addition, this gear-changing procedure leads to poor driving comfort.
It is also generally known to determine the resistance to travel and the force transmission between the tires and the roadway as disclosed in Zomotor, Adam; Fahrwerktechnik: Fahrverhalten; Wurzburg: Vogel, 1987; ISBN 3-8023-0774-7; pp. 17ff.
An object of the present invention is to achieve a method for controlling the drive slip in such a way that, in the case of a need to act on setting parameters of the internal combustion engine of a motor vehicle, this action takes into account insofar as possible, to take the way in which the engine torque transmitted from the internal combustion engine depends on a change to the setting parameters of the internal combustion engine due to the instantaneous operating condition of the internal combustion engine. In addition, the method should be as universally usable as possible and permit the simplest possible adaptation to changes in the internal combustion engine and/or the driving mechanism.
The present invention achieves those objects in a method in which the maximum drive torque which can be transmitted by the wheels of the motor vehicle is determined in a, calculating unit. A signal, which directly represents the determined maximum transmittable drive torque, is emitted from the calculating unit to the input of a central control device, and the central control unit is configured such that at least one setting parameter of the internal combustion engine is altered as a function of the signal representing the determined maximum transmittable drive torque to reduce the engine torque transmitted from the internal combustion engine.
Further advantages of the present invention relative to the known state-of-the-art comprise the fact that a simplified interaction of various components for controlling the different parameters is provided by allocating priorities to the output signals of these components so that no contradictory action by several of these components on setting parameters takes place. The priorities of the output signals of these components are determined by the importance of the components to the operational safety of the motor vehicle.
The method of the present invention is based on the fact that, when excessive slip occurs on at least two driven wheels on different sides of the vehicle, the maximum drive torque which can be transmitted between these driven wheels and the roadway surface is determined. The required torque to be transmitted from the internal combustion engine is then determined by taking account of the axle and gearbox transmission ratios. A required rotational speed of the internal combustion engine is derived from the dynamic wheel radius R.sub.dyn, the motor vehicle speed v.sub.v and the axle and gearbox transmission ratios. This required rotational speed and the required torque are supplied to a central control device of the internal combustion engine which causes the actual rotational speed and the actual engine torque of the internal combustion engine to achieve these required values by adjusting the setting parameters of the internal combustion engine.